This invention generally relates to electrical contact materials, more particularly to electrical contact materials having deposits that are laid down in series, the deposits being such that the electrical contact materials exhibit especially advantageous contact resistance and exceptional resistance to corrosion, while at the same time minimizing the use of gold or other noble metal.
Copper is well known and has long been used as an electrical contact material because of its very good electrical conductivity properties, although it is also well known as being subject to severe corrosion deterioration, making copper alone unsuitable for use as an electrical contact that cannot tolerate any significant build up in electrical resistance, such as contacts that are used in low energy, high performance electrical contact/connector applications including leads for transistor packages, leads for hybrid circuit packages, printed wiring board termination contacts, pins and sockets, and the like. Special problems arise when providing low force contacts or electronics connectors that have critical tolerances and must be of especially high reliability and consistency, especially consistency of contact resistance. This invention relates to these types of high technology contacts or connectors.
Prior improvements were made in these type of contacts by adding a layer of highly conductive yet corrosion resistant noble metal, usually a precious metal such as gold, plated over the copper substrate; however, this approach does not provide the ideal solution because high rates of corrosion and contact resistance deterioration are still exhibited when the contacts are subjected to severe environments. In such environments, there is a migration between the base metal and the noble or precious metal at the pore sites of the noble metal, characterized by a degradation mechanism that involves migration of the base metal corrosion products onto and across the noble metal surface. This migration and corrosion problem is reduced somewhat by providing a particularly thick gold layer, which is unsatisfactory for many applications because this approach significantly increases the cost of these electrical contacts.
Efforts to avoid the use of substantial quantities of gold have included over-plating the base metal with an electroless nickel deposit that is of a density and thickness adequate to significantly reduce corrosion of the base metal by providing a thick and dense intermediate electroless nickel layer. Unfortunately, the typical thick and dense electroless nickel layer is of relatively low conductivity when compared with metals such as copper and gold, thereby impairing the overall electrical contact resistance properties of the connector or contact. Additionally, many electroless nickel deposits themselves tend to oxidize, thereby further deteriorating the electrical contact resistance properties of the overall contact material.
Previous attempts to improve on the approach of overlaying a copper base with an electroless nickel deposit or other interior layer included plating a layer of noble or precious metal such as gold over these internal layers. This represented an effort to reduce the oxide buildup associated with the internal layer while enhancing conductivity. This approach has not been particularly successful except in those instances where the internal or electroless nickel layer is relatively thick in order to substantially reduce migration and where the gold layer is also relatively thick in order to lower the contact resistance to levels that are acceptable according to industry standards. This approach brings with it the disadvantage of being expensive due to the amount of gold needed and of experiencing bonding problems to the extent that there tends to be some separation between the layers of these electrical contacts, especially when they are subjected to deforming or bending forces.
The present invention advances the electrical contacts art by providing an electroless nickel barrier layer between the conductive substrate layer, usually copper, and the noble or precious metal layer, usually gold, which electroless nickel barrier layer assists in providing electrical contacts or conductors that exhibit superior contact resistance properties and exceptional resistance to corrosion over extended periods of time while simultaneously minimizing the quantity of noble or precious metal needed as the overlayer deposit. By the present invention, there is provided an electroless nickel barrier layer deposit that exhibits exceptional ability to diminish the tendency of metals, particularly copper, to migrate thereacross, this barrier layer being an electrical contact material that exhibits desirable electrical properties and which retains those properties under severe exposure conditions. The electrical contact material according to this invention also exhibits excellent bondability both initially and after storage and use for extended time periods.
It is accordingly a general object of the present invention to provide improved electrical contact materials.
Another object of the present invention is an improved multi-deposit electrical contact material that adheres to industrial contact resistance standards for precious metal contacts.
Another object of this invention is to provide an improved electrical contact material having a contact resistance of a magnitude generally on the order of 5 milliohms, which contact resistance is maintained for substantial periods of time under severe exposure conditions.
Another object of the present invention is to provide an improved multi-component deposit that exhibits excellent electrical conductivity which remains relatively constant over extended time periods, even under severe exposure conditions.
Another object of the present invention is to provide an improved electrical contact product that possesses excellent bondability and electrical properties which are retained for extended time periods.
Another object of this invention is to provide an electrical contact material that exhibits and retains excellent bondability to conductors by procedures such as diode bonding and ultrasonic bonding.
Another object of the present invention is to provide an improved bath system or composite bath for providing multiple, serial baths for preparing improved electrical contact materials.